Dumb Mid-Power Question

Dumb Mid-Power Question

Post by Mick Spence » Wed, 03 Nov 1999 04:00:00



While perusing the list of NAR/Tripoli certified engines, I noticed that
once you get up into the F's and G's, there are note only a few
different motor diameters, but also a bewildering variety of motor
lengths.  Since I've never built a kit that uses one of these motors, I
don't really know anything about the motor mounts, but in general, how
do you accommodate all those different motor lengths?  TIA.
---Mick
 
 
 

Dumb Mid-Power Question

Post by Peter Cl » Wed, 03 Nov 1999 04:00:00


Quote:
>how do you accommodate all those different motor lengths?

Question founded on years of putting the thrust ring/engine block **in the
rocket.**  Very simple answer: the thrust ring is on the nozzle end of the
**motor**, either built in or built up with tape.  Once you start doing it for
the big ones, you'll be doing it for the small ones too before you know it.

The other option is to put a regular engine block in the rocket for the longest
motor and use spacers for shorter ones, but I hardly ever see that anymore.

You have a few more things to get used to before this is all over. Metal
reloadable motors, mechanical motor retension, ignition delays, aftermarket
ignitors, a whole list. Relax. Enjoy it.  Have fun.
Peter W. Clay NAR 18619 SR L1
All control is in the hands of those who know.  Will they help us grow...?

 
 
 

Dumb Mid-Power Question

Post by Joe Pfeiffe » Wed, 03 Nov 1999 04:00:00


Quote:

> While perusing the list of NAR/Tripoli certified engines, I noticed that
> once you get up into the F's and G's, there are note only a few
> different motor diameters, but also a bewildering variety of motor
> lengths.  Since I've never built a kit that uses one of these motors, I
> don't really know anything about the motor mounts, but in general, how
> do you accommodate all those different motor lengths?  TIA.

You build around the longest motor you might want to launch with.
Then you either put spacers in the mount ahead of the motor you
actually use, or you glue a thrust ring around the aft end of the
motor so it pushes against the aft end of the mount (reloadables come
with the ring already in place).

You handle different diameters similarly; you design around the
biggest you might want, and build adapters for smaller.

(this is all from theory; I haven't gotten past the ``I wanna do that
some day'' stage either)
--
Joseph J. Pfeiffer, Jr., Ph.D.       Phone -- (505) 646-1605
Department of Computer Science       FAX   -- (505) 646-1002
New Mexico State University          http://www.cs.nmsu.edu/~pfeiffer

 
 
 

Dumb Mid-Power Question

Post by Lewis Garro » Wed, 03 Nov 1999 04:00:00


mid power retention is from the rear of the rocket. The aft closure on the
engine is wider than the motor mount tube. You need to insure the engine is
not ejected rearward due to the ejection charge. This can be done by various
methods the least of which is fricton/tape fit. If the engine is a single use
one you might have to build up a 'lip' on the nozzle end by using masking
tape wound to sufficient thickness to prevent the engine from moving forward
in the motor mount. People have come up with various motor retention schemes,
some of which can be found on http://www.rocketryonline.com. Based on the
fact that the retention is in the rear, the motor mount should be long enough
to support the longest motor for that mount size. More is better.
Quote:

> > While perusing the list of NAR/Tripoli certified engines, I noticed that
> > once you get up into the F's and G's, there are note only a few
> > different motor diameters, but also a bewildering variety of motor
> > lengths.  Since I've never built a kit that uses one of these motors, I
> > don't really know anything about the motor mounts, but in general, how
> > do you accommodate all those different motor lengths?  TIA.

 
 
 

Dumb Mid-Power Question

Post by Mark S. Morle » Wed, 03 Nov 1999 04:00:00


Let me follow up this fine answer with another stupid question. Why does
the MMT need to be as long as the motor?  Is it a real problem to have
part of either a SU or RMS motor with bits exenting forward of the MMT
inside the rocket??
]\/[ark

Quote:

> mid power retention is from the rear of the rocket. The aft closure on
> the engine is wider than the motor mount tube. You need to insure the
> engine is not ejected rearward due to the ejection charge. This can be
> done by various methods the least of which is fricton/tape fit. If the
> engine is a single use one you might have to build up a 'lip' on the
> nozzle end by using masking tape wound to sufficient thickness to
> prevent the engine from moving forward in the motor mount. People have
> come up with various motor retention schemes, some of which can be found > on http://www.rocketryonline.com. Based on the fact that the retention
> is in the rear, the motor mount should be long enough to support the
> longest motor for that mount size. More is better.

 
 
 

Dumb Mid-Power Question

Post by Rick Dickinso » Wed, 03 Nov 1999 04:00:00


On Tue, 02 Nov 1999 14:35:39 -0600, "Mark S. Morley"

Quote:

>Let me follow up this fine answer with another stupid question. Why does
>the MMT need to be as long as the motor?  Is it a real problem to have
>part of either a SU or RMS motor with bits exenting forward of the MMT
>inside the rocket??

Mark,

The motor mount needs to be long enough to ensure that the motor stays
straight.  It's easier to do that with a motor mount that's as long as the
motor.  Also, if your centering rings aren't perfect (and Murphy's laws of
engineering state that part tolerances *always* add unidirectionally), a
1/8" misalignment over a foot-long distance is much smaller of an angle
than the same 1/8" misalignment over a 4" distance.

Additionally, bigger rockets tend to have big interior volumes to
pressurize with your ejection charge.  Extending the motor mount as a
"stuffer tube" with a centering ring at the far end means that you have a
smaller volume to pressurize, and your "laundry" stays up closer to the
nose, where it's more likely to get ejected.

Of course, there is absolutely NO technical reason that you couldn't use a
1/2" long piece of motor mount tube at the tail end of your rocket as the
only motor mount.  Of course, you better make sure your motor fits tight
enough in that 1/2" long mount to be rigidly held *straight*, and that your
ejection charge is sufficient to blow your rocket's recovery system out....
It's just easier to do it with longer motor mounts.

 - RIck "Cardboard is cheap"***inson

--
The best laid schemes o' Mice an' Men,
Gang aft agley,
An' lea'e us nought but grief an' pain,
For promis'd joy!

 
 
 

Dumb Mid-Power Question

Post by Kevin Trojanowsk » Wed, 03 Nov 1999 04:00:00


You have plenty of answers to your question, but I'll add one more
item...

It's NOT a dumb question -- I had to ask the same one about 3 months
ago.

BTW, in case you're looking at North Coast Rocketry kits, their motor
mounts are 28.5mm.  Standard 29mm motors are a tight fit, so you need to
either modify or replace their motor mount tube to accomodate standard
motors.  Easily done; I've already done it twice.

-Kevin

 
 
 

Dumb Mid-Power Question

Post by Neil Tarasof » Thu, 04 Nov 1999 04:00:00


To all on ROL.

There is no such thing as a DUMB QUESTION, only DUMB SILENCE.

I teach this to all the kids I work with. We learn by ASKING QUESTIONS!

Neil Tarasoff

Quote:

> You have plenty of answers to your question, but I'll add one more
> item...

> It's NOT a dumb question -- I had to ask the same one about 3 months
> ago.

> BTW, in case you're looking at North Coast Rocketry kits, their motor
> mounts are 28.5mm.  Standard 29mm motors are a tight fit, so you need to
> either modify or replace their motor mount tube to accomodate standard
> motors.  Easily done; I've already done it twice.

> -Kevin

 
 
 

Dumb Mid-Power Question

Post by Peter Cl » Thu, 04 Nov 1999 04:00:00


Quote:
>To all on ROL.

>There is no such thing as a DUMB QUESTION, only DUMB SILENCE.

uh, Neil, this is r.m.r. ...on ROL there are lots of dumb questions...  
(Just kidding, Darrell, don't make me give the T-shirt back...)

The only dumb question is the one you knew you should have asked and didn't.

How to mount a midpower motor is **not** a dumb question.  Mounting one without
asking how **might** be a dumb thing to do.

Peter W. Clay NAR 18619 SR L1
All control is in the hands of those who know.  Will they help us grow...?

 
 
 

Dumb Mid-Power Question

Post by Gene Costanz » Thu, 04 Nov 1999 04:00:00


Mick, there are NEVER dumb questions here. If it weren't for rmr, I
wouldn't have progressed to the point where I am now.

Where's that, you query? Suffice to say that now I have accomplished the
following in my short, way-past-adolescence return to rocketry:

I lose some pretty hot rockets in style, some with the nicest amenities
on board...

I have the NICEST fleet (albeit short-lived) full of primed/unpainted
flyers...

Whenever I set up my rack of mod rocs and mid-powers at a launch next to
my 'ride', scores of flyers amble over and get a good snicker (I think I
need a new car, maybe?)

Somehow, whenever I get up to the launch line, I see club members draw
straws to see who HAS to LCO (I STILL think it's because they feel
honored to do so, although the 'winner' seems to drag his feet back to
the mike).

Whenever I mention on rmr or via email that I'm going to be attending a
launch, SOMEHOW someone goes out of their way to assure me that I'm
aware that the weather is "iffy". (Funny how that damn Weather Channel
site is never right, even for the same day, huh?) I'll never get over
that politeness.

I've posted many construction/finishing questions here and invariably, I
seem to get someone that tells me something that contrasts otherwise
"public" info (via anonymous email server). This makes me feel
one-of-a-kind. (BTW, I'm going to fly my Titebond-"resined" fiberglass
job as soon as the TB dries {around May 2000, I'd estimate} and thanks
for the tip to whomever sent it.)

Definitely can't beat the comraderie here, especially when you feel so
well-liked. Kinda makes me misty thinkin' about it...

 
 
 

Dumb Mid-Power Question

Post by Lewis Garro » Thu, 04 Nov 1999 04:00:00


Definitely ditto to this. When I was a BAR and only had worked with Estes
modrocs many moons ago, I only knew about engine retention via clips and
tape and a motor block in front of the engine. The transition to mid and
high power required learning some of the construction differences by asking
more experienced modelers and looking right here for answers.
Quote:

> You have plenty of answers to your question, but I'll add one more
> item...

> It's NOT a dumb question -- I had to ask the same one about 3 months
> ago.

> BTW, in case you're looking at North Coast Rocketry kits, their motor
> mounts are 28.5mm.  Standard 29mm motors are a tight fit, so you need to
> either modify or replace their motor mount tube to accomodate standard
> motors.  Easily done; I've already done it twice.

> -Kevin

 
 
 

Dumb Mid-Power Question

Post by Gene Costanz » Thu, 04 Nov 1999 04:00:00


OK, Lew, I'm waiting for the punch line <taptaptap>...

Don't forget all the wonderful advice *I* gave you that fateful night
(not to buy a 4" AMRAAM, etc...)  :-)>

 
 
 

Dumb Mid-Power Question

Post by Jerry Garci » Sat, 06 Nov 1999 04:00:00



Quote:

> Mick, there are NEVER dumb questions here. If it weren't for rmr, I
> wouldn't have progressed to the point where I am now.
>No, only dumb people like you George.  I laugh when you say

progressed.  You New York Jews bust me up.

Sent via Deja.com http://www.deja.com/
Before you buy.